Most of the user equipments (UE) today are equipped with multiple radio interfaces supporting access to wireless services over multiple radio access technologies (RATs). State of the art UEs are capable of running multiple Internet Protocol (IP) based applications simultaneously, possibly using several radio interfaces at the same time. In fact, such multi-homing capability is routinely supported by state of the art desktop or laptop computers running IPv6, but multi-homing is supported by state or the art (IPv4) operating systems as well.
Such multi-RAT devices may be engaged in cellular as well as in local adhoc communications sessions simultaneously. For example, a laptop computer or a personal digital assistant (PDA) can be part of a local adhoc network running a peer-to-peer file download or faster-than-real-time streaming, e.g. YouTube, session while running a real time voice or video session over a cellular interface.
In addition, UEs in terms of their maximum output power capability belong to a particular UE power class (PC). For instance in WCDMA (Wideband Code Division Multiple Access) networks, there are several UE power classes namely:                PC4=21 dBm        PC3bis=23 dBm        PC3=24 dBm        
For instance a WCDMA UE belonging to the PC3, which is the most commonly used power class, can operate at maximum output power equal to 24 dBm.
In LTE, hitherto there exists only one UE power class namely:                PC3=23 dBm        
Future LTE-Advanced and IMT-Advanced compliant user equipments may include further UE power classes.
Most electronic devices, including 3 GPP compliant UEs and devices equipped with multiple radio interfaces regardless of their power class must comply with international and various national RF regulatory requirements regarding overall power emission, radio frequency exposure and specific absorption rate (SAR). Although the specific requirements vary in different regions, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) sponsored by the World Health Organization (WHO) recommends guidelines based on consensus of existing scientific results and provides protection against non-ionizing radiation (NIR).
The ICNIRP SAR limits for IMT-2000 terminals are concerned with radiation exposure in the frequency range between 300 Hz and 300 GHz which is the ICNIRP definition of radio frequency (RF), including specific radiation thresholds, measurement methods and other technical details. For example the ICNIRP recommendations specify the so called averaging time (Tavg) which is the appropriate time interval over which the RF exposure is averaged for the purpose of determining compliance with the exposure limits. This averaging time can vary from region to region, but it is roughly in the order of several minutes. However, ICNIRP recommendations also concern short term (in the order of several seconds or tens of seconds) exposure limits that may be higher than those defined for Tavg thus allowing for higher exposures for a short duration of time.
It is important to note that the SAR regulations are concerned with the overall radiation limitation typically given in [Watts/body kg] irrespectively of the used radio interfaces, the number of simultaneously used RATs, accessed RATs, etc.
Another example is when a UE is operating in a frequency band, which is adjacent to or in close proximity of sensitive operations such as public safety systems (e.g. 700 MHz operation in US) or medical installations etc. In these or similar sensitive scenarios, in order to be compliant with the FCC regulatory constraint, the total UE transmit power over all the available radio interfaces, cellular and non-cellular, may have to be limited to a value lower than its maximum output power capability (i.e. power class level) or the total UE transmit power over all the available radio interfaces should at least not exceed its maximum output power capability provided that the regulatory requirements can be met.
A specific example is that of E-UTRA FDD (Evolved Universal Terrestrial Radio Access Frequency Division Duplex) frequency band 13, defined in E-UTRA specification 3GPP TS 36.101, which is adjacent to the public safely frequency band in 700 MHz range. Hence when operating in band 13 in order to limit emissions towards the public safety band in compliance with FCC regulations, the maximum transmit power of the LTE UE with power class 3, i.e. 23 dBm, has to be lowered by several dBs e.g. 10 dBm below the maximum power level. The actual reduction in the maximum transmit power depends upon the number of uplink transmitted resource blocks.